Vehicles, and in particular off-highway machinery such as wheel loaders, excavators, dozers, articulated vehicles, tractors, harvesters and similar heavy duty machines often operate with hydraulic, electro-hydraulic, and electric steering systems. Typically, the steering system receives a desired input to indicate a desired angular movement of the vehicle. The input could be provided via input means such as a joystick, a steering wheel, or the input could be derived from a GPS system or similar system for positioning and tracking. The input is converted into a signal for an actuator, e.g. a hydraulic cylinder, which moves a steered element, e.g. a wheel of a vehicle or a rudder of a ship. A steering system is disclosed e.g. in U.S. Pat. No. 6,668,967.
In a fully hydraulic steering, the actuator is typically supplied with a hydraulic flow via a steering unit which is connected to the steering input means. The steering unit often comprises a spool which is moved in dependency of the position of the input means. The spool opens or closes a path from a source of a hydraulic fluid under pressure to one out of two hydraulic ports of the actuator. A fluid flow in one of the ports forces the actuator to move in one direction and a fluid flow in the other port forces the actuator in an opposite direction, e.g. right or left. Typically, the input means is a steering wheel or a joystick. The input means is movable between a centre set-point and set-points on opposite sides of the centre set-point, e.g. corresponding to a right turn or a left turn. The steering unit is normally provided to activate the actuator to move in the right direction as long as the input means is to the right of the centre set-point and to move in the left direction as long as the input means is to the left of the centre set-point. In the centre set-point, the actuator is normally stopped, and the steered element is locked in the position where it was when the input means reached the centre set-point. This is sometimes referred to as open loop-steering. Alternatively, the actuator and thereby the steered element returns to a neutral position. Normally, the input means is biased towards the centre set-point which means that the user forces the input means away from the centre set-point, and when the user releases the grip in the joystick or steering wheel or otherwise let go of the input means, the input means returns to the centre set-point. This is sometimes referred to as closed loop-steering.
A change of acceleration, often called jerk, may be uncomfortable for the driver. In particular with respect to articulated vehicles, jerks may occur if the input means is moved to the centre set-point or to a right or left extreme set-point too fast, and to avoid such jerks, systems exist in which the steering unit is adapted to move the spool at a reduced speed. This naturally slows down the steering speed and may be annoying and even unsafe in certain cases.